Modern aircraft engines desire both the fuel efficiency of high-bypass turbofans and the high specific thrust of a low-bypass turbofan. Engine performance and engine efficiency are generally in conflict according to traditional engine architectures. High-performing engines tend to be less efficient, and highly efficient engines tend to lack the thrust capabilities of high performance engines. As a result of this conflict, an engine is usually designed to best meet requirements for its primary objective. Thus, military aircraft generally utilize high-performing engines while commercial aircraft use high-efficiency engines.
Advances in engine architecture suggest that adding a second bypass stream to a traditional turbofan can provide significant benefits. The core engine flow is considered the primary or first stream, an inner bypass stream is the second stream, and an additional second bypass stream is the third stream. An engine having three airflow streams can be independently modulated to provide additional core flow (higher-performance) or additional bypass flow (higher-efficiency). In order to achieve these cycle benefits, each of the flows must exit via one or more nozzles that provide efficient thrust generation and reduces any drag penalty.